The rapid identification of molecular species has many applications in the areas of science and technology. The determination and measurement of harmful pollutants in the environment also has gained increasing importance as government agencies require industries to meet pollution control standards based on the best available testing technologies. The development of inexpensive equipment that can provide a rapid measurement of chemical species in environmental samples can thus have a wide-ranging application.
Various spectroscopic techniques monitor the interaction of laser light with a sample by measuring either transmitted or absorbed laser light as a function of wavelength. Many absorption techniques such as frequency modulation and wavelength modulation spectroscopy estimate species according to the derivative of the spectra. These techniques are best suited to detecting small molecules with well defined spectral features as they are not capable of discriminating the broad spectral features of large molecules. The difference between the spectra of a large molecule, such as toluene, and a small molecule, such as NO2, are illustrated in FIG. 1. In comparison with small molecules, the spectral features of large molecules generally include fine spectral features over a broad spectral range. It is difficult or impossible for many existing laser-based spectroscopic techniques to quantitatively speciate mixtures of such large molecules.
Photoacoustic spectrometers, in contrast to most other techniques, analyze a sample according to heat absorption and the resulting pressure waves generated within the sample. Photoacoustic spectrometers are described, for example, in U.S. Pat. No. 3,948,345 to Rosencwaig, incorporated herein by reference. In photoacoustic spectroscopy, a tunable light source is passed through a sample contained in an enclosed cell. As the wavelength of the light source is varied, the sample absorbs light according to it absorption spectra. Absorbed light is converted into heat within the sample that is detectable as an increase in pressure of the contained sample. The photoacoustic spectrum of the sample is the variation of pressure oscillations in a sample with the wavelength of light from the light source. The ability to speciate mixtures of complex molecules requires a light source having an output that is both tunable over the absorption wavelength range of the molecules and narrow enough to capture fine spectroscopic features of the particular molecules. In addition, sufficient power must be available to produce measurable pressure oscillations or pulses in the sample and distinguish these pulses from background noise. Photoacoustic spectrometers are capable of measuring concentrations of complex molecular species at concentrations of parts per billion, and thus have great potential for the rapid speciation of complex toxic compounds in the air.
Of concern for environmental measurements is the detection of volatile organic compounds (VOCs). The optimum wavelength ranges for detecting VOCs is generally 3–5 μm and 8–12 μm, where atmospheric transmission is good and where functional organic groups, such as the fundamental stretch mode of C—H, strongly absorb. At present there are several promising sources in the mid-range infrared range of 3–5 μm. The most promising sources in the 8–12 μm range are the CO2 lasers and the quantum cascade diode lasers. The former, however, is only tunable over about 40 discrete lines in the 9 to 11 μm range. The latter are only tunable over about 10 cm−1 per device.
Tunable light in the mid-range infrared can be generated with available light sources through the interaction of laser light with non-linear optical materials. Typically, the output wavelength is varied by changing some physical property of the non-linear material, such as its temperature or orientation. This technique for generating tunable light is particularly promising for environmental uses, since it has the potential to be robust and relatively maintenance-free. Higher output powers and stable output wavelength can be generated non-linear materials by incorporating them into an optical oscillator.
A non-linear material that is particularly useful for spectroscopy and chemical sensing is periodically poled lithium niobate (LiNbO3), or PPLN. U.S. Pat. No. 5,434,700 to Yoo, incorporated herein by reference, describes the operation of optical wavelength converters constructed of materials having non-linear optical properties. The non-linear properties of a PPLN crystal can be changed by changing the material temperature or by adjusting the orientation of light relative to the non-linear material structure, such as by rotating the material relative to the incident light path, or by having a material with varying structures and by moving the material so that different portions of these varying structures intercept the incident light.
While strides have been made in the development of photoacoustic spectrometers, prior art systems have limitations that hinder their use for environmental applications. One of the major limitations is the inability of prior art systems to conduct real-time measurements of mixtures of complex organic compounds. To accomplish this, the light source must be narrow and finely tunable (either continuously, or in steps of a fraction of a wave number) over a broad range (hundreds of wavenumbers). In addition, it must be capable of being used at the place where the environmental measurement is to be made that is it must be portable so that is useful in the field.
Prior art systems typically use lasers having an output in the several watt range to drive non-linear materials. For example, such systems have used neodymium-vanadate (Nd:Vanadate) pump lasers operating at about 1 μm and generating sufficient power to induce non-linear effects in non-linear materials, such as PPLN. Typically the non-linear material in located in an optical parametric oscillator (OPO) that is tuned to produce light of a wavelength different from the pump laser. While these systems produce usable IR light, there are many problems in adapting them for portable applications, such as real-time environmental measurements. Prior art systems typically have limited tuning capabilities and require large amounts of external power, making it difficult to include them in portable photoacoustic spectrometers.
What is needed is an improved photoacoustic spectrometer which has a laser system that operates at high efficiency and generates light with a beam profile that efficiently couples into an OPO, which is be capable of speciating gaseous mixtures of complex organic molecules, and which is robust and portable.